Implantable medical devices

ABSTRACT

A medical device includes a balloon catheter having an expandable member, e.g., an inflatable balloon, at its distal end and a stent or other endoprosthesis. The stent is, for example, an apertured tubular member formed of a polymer and is assembled about the balloon. The stent has an initial diameter for delivery into the body and can be expanded to a larger diameter by inflating the balloon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims benefit of priority fromU.S. patent application Ser. No. 12/880,483, filed Sep. 13, 2010, whichis a divisional of U.S. patent application Ser. No. 10/958,435, filedOct. 5, 2004, now U.S. Pat. No. 7,794,494, which is acontinuation-in-part of, and claims benefit of priority from U.S. patentapplication Ser. No. 10/683,314, filed Oct. 10, 2003, now abandoned,which claims the benefit of priority from U.S. Provisional PatentApplication Ser. No. 60/418,023, filed Oct. 11, 2002, U.S. ProvisionalPatent Application Ser. No. 60/488,323, filed Jul. 18, 2003, U.S.Provisional Patent Application Ser. No. 60/419,506, filed Oct. 18, 2002,U.S. Provisional Patent Application Ser. No. 60/466,401, filed Apr. 29,2003, and U.S. Provisional Patent Application Ser. No. 60/488,590, filedJul. 18, 2003, the contents of each application above is herebyincorporated by reference herein in its entirety.

TECHNICAL FIELD

This invention relates to implantable medical devices and methods ofdelivering the same.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesisinclude stents and covered stents, sometimes called “stent-grafts”.

An endoprosthesis can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, for example, so that it can contact thewalls of the lumen.

Prostate enlargement, also known as benign prostate hyperplasia orbenign prostate hypertrophy, is a common affliction among older men. Thecondition involves swelling of the prostate. The prostate surrounds theurethra, or urinary tract, and enlargement of the prostate may restrictpassage of urine from the bladder towards the urethra. Benign prostatehyperplasia is uncomfortable because it makes urination difficult orimpossible. The condition is also dangerous because it can lead toinfection of the bladder and kidneys.

Prostate enlargement can be treated with surgery known as resection.Resection can be accomplished by cutting away a large portion of theprostate gland. Prostate enlargement can also be treated with heattreatment, cold treatment, or ablation.

Sometimes a restricted urethra can be treated with a prostatic stent tosupport the urethra and keep it open despite pressure from the enlargedprostate. A prostatic stent may be implanted permanently or as aninterim solution.

SUMMARY

The invention relates to implantable medical devices, for example, astent including a polymer.

In one aspect, the invention features a medical device. The medicaldevice includes a balloon catheter having an expandable member, e.g., aninflatable balloon, at its distal end and a stent or otherendoprosthesis. The stent is an apertured tubular member formed of apolymer and is assembled about the balloon. The stent has an initialdiameter for delivery into the body and can be expanded to a largerdiameter by inflating the balloon. The polymer does not flowsubstantially during expansion and substantial stress relaxation orcreep does not occur so that the geometry of the stent is maintained.

In another aspect, a tubular endoprosthesis including a polymer body isprovided and delivered into a body lumen. The endoprosthesis is expandedin the body lumen under conditions of expanding pressure and temperatureso that the wall thickness of the polymer body is substantiallymaintained.

In another aspect, a polymer tube is formed to a first, large diameter.An aperture pattern is cut into the tube wall. The polymer iscrosslinked or crystallized. The polymer tube is deformed to a second,small diameter. The polymer tube is expanded in a body lumen to adiameter larger than the second diameter by application of pressure andheat.

In another aspect, a polymer tube is formed to a first, small diameter.An aperture pattern is provided in the tube wall. The polymer iscrystallized or crosslinked. The tube is expanded in a body lumen byapplication of pressure and heat.

In another aspect, an implantable medical apparatus includes an elementoperable for movement within the body by mechanical force applied to theelement. The element includes a polymer having a melt or glasstransition temperature in the range above body temperature to about 50°C. or 60° C. and exhibiting a plateau in a plot of storage modulus as afunction of temperature at melt or glass transition. In embodiments, theelement is a stent. The stent may be generally a tubular body thatincludes an apertured wall. The stent may be operable for expansion froma first, smaller diameter to a second larger diameter for implantationin a lumen. The thickness of the stent wall varies by about 1% or lessbetween the first and second diameter.

In another aspect, the invention features a medical device including apolymer having a melt or glass transition temperature above bodytemperature and exhibiting an approximate plateau in a plot of storagemodulus as a function of temperature at melt or glass transition. Themelt or glass transition temperature may be, for example, above about37° C. The medical device may undergo a triggerable event at about theplateau. The triggerable event may be, for example, a change in theflexibility, a change in the porosity, a change in the coefficient offriction or a change in the surface roughness. The medical device maybe, for example, a stent that has a portion that has a collapsedposition that can be reverted to an expanded position by a triggersubsequent to insertion into the body.

Aspects may include one or more of the following features. The polymerbody, optionally, includes apertures. The polymer body has a ratio ofaperture open area to wall area of about 0.5 or more or 0.7 or more. Theendoprosthesis is expanded by simultaneously applying an expandingpressure and heat to the endoprosthesis. The polymer body is heatedabove the melt or glass transition temperature of polymer in the polymerbody. The polymer body is elastomeric at the melt or glass transitiontemperature. The polymer is elastomeric at body temperature. The polymeris crystalline. The polymer is crosslinked. The polymer is radiationcrosslinked. The melt or glass transition temperature is about 40 to 50°C. The melt or glass transition temperature has a transition range ofabout 5° C. or less. The polymer exhibits a plateau in the melt or glasstransition range in a plot of storage modulus as a function oftemperature. The polymer body includes a drug, radiopaque agent ormagnetic heating agent. The polymer is a shape memory polymer, e.g.capable of remembering a smaller diameter configuration after expansion.The polymer is, for example, polynorbornene, polycaprolactone, polyenes,nylons, polycyclooctene (PCO), blends of PCO and styrene-butadienerubber, polyvinyl acetate/polyvinylidinefluoride (PVAc/PVDF), blends ofPVAc/PVDF/poly-methylmethacrylate (PMMA), polyurethanes,styrene-butadiene copolymers, polyethylene, trans-isoprene, blends ofpolycaprolactone and n-butylacrylate, PVC, e.g., plasticized PVC, andblends thereof. An expansion pressure of about 1 atm or more is applied.The endoprosthesis is delivered on a catheter. The endoprosthesis isdelivered to a site of occlusion and the site is simultaneously dilatedwhile expanding the endoprosthesis. The endoprosthesis is delivered to asite of lumen curvature and the endoprosthesis is expanded at the site.The endoprosthesis is delivered to a vascular lumen. The endoprosthesisis delivered adjacent (into) the prostate.

Aspects may include one or more of the following. A heat applicatorapplies heat to the stent during inflation of the balloon to expand theballoon to the expanded diameter. The polymer has a melt or glasstransition temperature in the range of about 40 to 50° C. and a modulusat the melt or glass transition temperature sufficient to maintain thestent geometry or under application of pressure and/or heat. The polymerexhibits a plateau in the storage modulus in the range of melt or glasstransition temperatures. The stent has a wall thickness of about 0.005to 5 mm. The stent has an initial unexpanded inner diameter in the rangeof about 1 mm to 5 mm. The stent has an expanded inner diameter of about1 mm to 20 mm. The stent may be expandable to about 100% or 400% or moreof the initial inner diameter. An example of a coronary stent has aninitial inner diameter of about 2 mm, and expanded inner diameter ofabout 4 mm and the wall thickness is about 0.005 mm to 0.1 mm. The stentcan be in the form of a tube including aperture areas provided in thetube. The aperture are in the shape of elongate slots, e.g., when thestent is in the small diameter condition. The apertures have a dimensionof about 1 mm or less in the small diameter condition. The apertures arein the shape of diamond-like openings, e.g. when the stent is in anexpanded condition. The stent can be a wire-form formed of one or morefilaments configured to generally define a tube.

Embodiments may include one or more of the following advantages. Aballoon expandable stent made of a polymer can be provided thatmaintains the integrity of the stent geometry on expansion and heating.Maintenance of stent geometry is desirable since geometry affects, forexample, the resistance to compression in the body and a predictablegeometry is important to avoid irregular surfaces, kinking, orextensions of material into the body lumen which can interfere with theflow of body fluid. The polymers can be elastomers that have melting orglass transitions at temperatures safe for use in the body and exhibitelastomeric properties at both the melted or glass transition stage andthe solid or crystalline phase. The stent body exhibits high resistanceto inward compressive forces when the polymer is in the solid orcrystalline phase. The elastomeric nature of the polymer in the meltedor glass state enhances the ability to maintain geometry as the stent isexpanded. For example, the polymer exhibits minimal flow duringexpansion and the thickness of the stent remains substantially constant.Elastomeric properties in the crystalline or solid state enhance theability to conform to torturous curvature in narrow body lumens. Highcompression resistance allows the stent to maintain the body lumen openand resist occluding forces such as elastic recoil or the growth ofthrombus from the vessel wall.

In another aspect, the invention features a polymeric stent having aportion that has a collapsed position that can be reverted to anexpanded position by heating above a first temperature subsequent toinsertion of the stent into a cavity or lumen. The stent may be in theform, for example, of a coiled elongated element (for example, a strand,a tape or a flattened tube). The stent may be further heated to a secondtemperature that is higher than the first temperature and removed as asubstantially uncoiled element. When the stent is in the form, forexample, of a coiled elongated flattened tube, the flattened tube mayinclude a central opening that includes a medicament that can bereleased by the inserted stent. In some implementations, the medicamentis compounded into the plastic or is a coating on the plastic. In someimplementations, the portion is at an end of the stent and the portionis flared or stepped. In other implementations, the portion includesless than 50% of the length of the stent.

In another aspect, the invention features a polymeric stent in the formof a coiled elongated element, and having a portion that has a collapsedposition that can be reverted to an expanded position by heating above afirst temperature subsequent to insertion of the stent into a cavity orlumen. When the stent is heated to a second temperature higher than thefirst temperature, the modulus of the element lowers sufficiently thatthe stent can be removed from the cavity or lumen as a substantiallyuncoiled element.

In yet another aspect, the invention features a method of treating anon-vascular cavity or lumen. The method includes inserting a polymericstent having a portion in a collapsed position that can be reverted, byheating, to an expanded position. Following insertion, the stent isheated sufficiently to revert the portion in the collapsed position tothe expanded position. The method may further include heating the stenthaving the portion in the expanded position sufficiently to soften thestent, and removing the softened stent from the cavity or lumen.

The stent may be, for example, a coiled elongated element (for example,a rod, a tape or flattened tube) and the heating of the stent prior toremoval allows the stent to be removed in a substantially uncoiledstate. This method provides ease of removal, for example, for removingprostatic stents that have been inserted on an interim basis. Theheating may be performed, for example, on a delivery tube.

In some embodiments, the portion of the stent is at the end of the stentand may be flared when in the expanded position. In other embodiments,for example, the portion of the stent is not at an end of the stent.

In still another aspect, the invention features a polymeric stentincluding metal particles. A portion of the stent has a collapsedposition that can be reverted to an expanded position by heating. Theheating may be performed using inductive heating to revert the portionin the collapsed position to the expanded position.

In another aspect, the invention features a stent having an exteriorsurface that includes a plurality of protruding elements that extendoutwardly from the surface. The protruding elements may be useful inhelping the stent retain its position, for example, after insertion intothe prostatic urethra.

In some embodiments, the protruding elements are formed of monofilament.The monofilament may include a plurality of constrictions along itslength.

In some implementations, the stent is a polymeric stent and the stenthas a portion that has a collapsed position that can be reverted to anexpanded position by heating above a first temperature subsequent toinsertion of the stent into a cavity or lumen.

In another aspect, the invention features an implantable endoprothesisincluding a tubular member that includes a polymeric material. Thetubular member has a wall having a first transverse dimension and afirst longitudinal length, measured when at the first transversedimension, sized for delivery into a lumen. Upon exposure to an elevatedtemperature, the tubular member can be expanded to a second transversedimension that is at least about fifty percent larger than the firsttransverse dimension within the lumen, the first and second transversedimensions being measured from an outer surface of the wall of thetubular member. The tubular member also has a second longitudinallength, measured when at the second transverse dimension. Afterexpansion from the first transverse dimension to the second transversedimension, the second longitudinal length decreases by less than aboutfifty percent, measured relative to the first longitudinal length.

In some implementations, the tubular member has a wall thickness,measured from an inner surface of the wall to the outer surface of thewall, and the wall thickness decreases by greater than about twentypercent, e.g., greater than about thirty percent, greater than aboutfifty percent, greater than about seventy-five percent, or greater thaneighty-five percent, after expansion from the first transverse dimensionto the second transverse dimension.

In some embodiments, after expansion from the first transverse dimensionto the second transverse dimension that is at least about forty percentlarger than the first transverse dimension, e.g., seventy-five percentlarger than the first transverse dimension, the second longitudinallength decreases by less than about twenty percent, measured relative tothe first longitudinal length.

The tubular member can be, for example, approximately circular intransverse cross-section, or the tubular member can have othertransverse shapes, e.g., non-circular, e.g., elliptical.

In some embodiments, the polymeric material has a softening temperaturefrom about 40° C. to about 60° C., e.g., 45, 50, 55, or 58° C. Thepolymeric material can be cross-linked, non-cross-linked, a shape memorypolymer, or a non-shape memory polymer. In some instances, the polymericmaterial is, for example, polycyclooctene (PCO), a styrenic elastomer, astyrenic block copolymer, a styrene-butadiene rubber, a polyolefin,trans-isoprene, or blends of these materials. The polymeric material caninclude a filler, e.g., a radio-opaque agent, e.g., bismuth carbonate,barium sulfate, or mixtures of these materials. Other fillers includes,for example, a thermal conductor, e.g., a boron nitride, other ceramics,or a metal.

In some implementations, the tubular member is, for example,substantially straight before it is expanded. In specific embodiments,the tubular member is curved after it is expanded and/or the outersurface of the wall of the tubular member includes a protruding elementthat extends outwardly from the outer surface after the tubular memberis expanded.

In some embodiments, the wall of the tubular member includes at leastone aperture defined therein.

In some implementations, the plastic has a elastic modulus of greaterthan about 50,000 psi, e.g., greater than about 75,000, greater thanabout 150,000, greater than about 250,000, or greater than about 500,000psi.

In another aspect, the invention features a method of treating apatient. The method includes placing the endoprosthesis just discussedon a delivery system. The delivery system then is used to deliver theendoprosthesis a lumen, e.g., a pulmonary lumen, an esophageal lumen, abiliary lumen, an enteral lumen, a ureteral lumen, and a urethral lumen.The endoprosthesis then is heated and expanded within the lumen. In aspecific implementation, the delivery system includes a ballooncatheter.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features, andadvantages of the invention will be apparent from the description anddrawings and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are side views of a portion of a stent in a smalldiameter and expanded condition, respectively.

FIGS. 1C and 1D are cross-sectional views of a portion of a stent in asmall diameter and expanded condition, respectively.

FIGS. 2A-2C illustrate delivery of a stent into a body lumen.

FIG. 3 is a plot of storage modulus as a function of temperature.

FIG. 3A is a plot of storage modulus as a function of temperature forsamples of PCO with varying degrees of crosslinking

FIG. 3B is a WAXS 2θ plot for samples of PCO with varying degrees ofcrosslinking.

FIG. 4 illustrates manufacture and use of a stent.

FIG. 5 illustrates manufacture and use of a stent.

FIG. 6 is a perspective view of a stent with an end in an expandedposition.

FIG. 7 is a side view of the stent shown in FIG. 6.

FIG. 8 is a side view of the stent shown in FIG. 6 with the end in acollapsed position.

FIG. 8A is a graph of heat flow as a function of temperature for severalPOSS polyurethanes.

FIG. 8B is a graph of storage modulus as a function of temperature forseveral POSS polyurethanes.

FIG. 9 is a perspective view of a wrapping fixture.

FIG. 10 is a cross-sectional view of a restricted prostatic urethra.

FIG. 11 is a cross-sectional view illustrating delivery of a stent tothe prostatic urethra.

FIG. 12 is a cross-sectional view of a prostatic stent deployed in theurethra.

FIG. 13 is a side view of an alternative delivery system.

FIG. 13A-13B are side views of an alternative delivery system.

FIG. 14 is a cross-sectional view illustrating removal of a prostaticstent.

FIG. 15 is a graph of storage modulus (E′) VS temperature for PCO.

FIG. 16 is a graph of storage modulus (E′) VS temperature forPVAc/PVDF/PMMA blends.

FIG. 17 is a side view of an alternative stent with two ends portions inexpanded positions.

FIG. 18 is the stent shown in FIG. 17 with ends in collapsed positions.

FIG. 19 is a side view of an alternative stent with three portions inexpanded positions.

FIG. 20 is a side view of an alternative stent made with a flattenedtube.

FIG. 21 is a cross-sectional view of the stent shown in FIG. 20, takenalong 21-21.

FIG. 22 is a side view of an alternative stent made with a tape.

FIG. 23 is a perspective view of a stent with a plurality of protrudingelements, the end of the stent is in an expanded position.

FIG. 24 is a side view of the stent shown in FIG. 23.

FIG. 25 is a side view of the stent shown in FIG. 23 with the end in acollapsed position.

FIG. 26 is a perspective view of an alternative stent with a pluralityof protruding elements, the end of the stent in an expanded position.

FIG. 27 is a side view of the stent shown in FIG. 26.

FIG. 28 is a side view of the stent shown in FIG. 26 with an end in acollapsed position.

FIGS. 29 and 30 are perspective views of a tubular stent in anunexpanded state and in an expanded state, respectively.

FIGS. 31 and 32 are perspective views of an elongated, tubular stent inan unexpanded state and in an expanded state, respectively.

FIG. 33 is a perspective view of a curved tubular stent in an expandedstate.

FIG. 34 is a perspective view of a tubular stent in an expanded statethat has flared ends.

FIG. 35 is a perspective view of an elongated tubular stent having anouter surface that includes a plurality of projections.

FIG. 36 shows DSC traces of PLA (top) quenched from T=180° C. or(bottom) annealed at T=110° C. for 1 hr.

FIG. 37 shows DSC traces for PLA/PVAc blends following annealing for 1hour at T=110° C. A heating rate of 10° C./min was employed. PLA weightpercent is indicated with each trace.

FIG. 38 shows glass transition temperatures measured following quenchingof the PLA/PVAc blends (solid points). Solid line is best fit to the Foxequation, 1/T_(g)=w_(a)/T_(g) ^(a)+w_(b)/T_(g) ^(b).

FIG. 39 shows tensile storage modulus versus temperature for a range ofPLA/PVAc blends whose composition is indicated in the plot.

DETAILED DESCRIPTION

Referring to FIGS. 1A and 1B, a stent 10 includes a polymer body 12generally defining a tube. The stent includes open areas 14. Referringparticularly to FIG. 1A, in a small diameter condition, such as fordelivery to a treatment site in the body, the open areas are relativelysmall and defined as slots cut through the wall of the stent. Referringparticularly to FIG. 1B, in the expanded condition, the slots arewidened to diamond-like shapes. The expansion mechanism of the stentutilizes a deformation (arrows) of the wall material about the openareas. As illustrated, the expansion results in a generally regular,symmetric, geometric pattern that resists and distributes inwardcompression of the stent by forces imposed by the lumen wall. Referringas well to FIGS. 1B and 1D, the wall thickness T of the stent does notsubstantially change upon expansion from the small diameter collapsed(T_(C)) condition to the expanded condition (T_(E))(T_(C)=T_(E)). Thepolymer does not substantially flow or thin out on expansion, so that areliable expansion geometry and wall thickness can be achieved. Otherstent constructions are suitable. For example, filament-form stents inwhich filaments of polymer material are arranged to define a generallytubular structure can be used. Open areas are defined between thefilaments. An example of a stent design including helical filaments isprovided in Wallsten, U.S. Pat. No. 4,655,771. Suitable aperture walldesigns are also described in Palmaz U.S. Pat. No. 4,733,665. Anothersuitable arrangement is exemplified by the Express stent, commerciallyavailable from Boston Scientific, Natick, Mass.

Referring to FIGS. 2A-2C, the delivery of a stent into the body isillustrated. The stent is delivered utilizing a catheter 24, whichincludes a catheter body 25 that carries a balloon 26 at its distal end.At the proximal end, the catheter includes an inflation apparatus 28such as a syringe or pump apparatus which can be used to inject andcirculate inflation fluid into the catheter, where it is directed by alumen to the interior of the balloon so that the balloon can beinflated. In addition, the inflation apparatus can include a heatingapparatus 30, to heat the inflation fluid directed to the balloon. Thecatheter is delivered into a vessel 20 including body lumen 21 in apatient to the site of an obstruction 31 typically utilizing a guidewire32. The guidewire 32 extends through a lumen within the body 25 of thecatheter.

Referring particularly to FIGS. 2A and 2B, the stent 10 is positionedover the inflatable balloon 26. Referring particularly to FIG. 2A fordelivery into the body, the balloon is initially in a small diameterdeflated condition. The stent is in a small diameter condition over theballoon. Referring particularly to FIG. 2B, when the treatment site isreached, the balloon is inflated by actuating the inflation apparatus28. The inflation fluid is heated to heat the polymer body of the stent10. By providing outward radial force while heating the stent, the stentis expanded into contact with body lumen 21. The stent can be expandedsimultaneously with the widening of the obstructed region. Afterexpansion to the desired diameter, the temperature of the inflationfluid is typically decreased to reverse the softening of the stent body10. Referring particularly to FIG. 2C, after the temperature of thestent has been reduced in this manner, it remains implanted in thevessel to resist vessel recoil and reduce restenosis after the balloonis deflated and the catheter is removed from the body.

Suitable polymers include those that maintain stent geometry underexpansion conditions, allowing for intricate stent geometries such asapertured tubes having high open area to wall ratios. At temperaturesabove body temperature and under conditions of radial expandingpressure, the stent can be expanded without fracture or substantialirreversible stress relaxation or creep. Typically, the stent is heatedto or above the melt or glass transition temperature during expansion.In this condition, the polymer is in a softened state. In this state,the polymer can be predictably deformed, typically about apertureregions during expansion. In addition, the soft condition permits properapposition of the stent to the lumen wall without kinking and withoutdamage due to excessive stiffness, which could straighten the lumen fromits native curvature and lead to dissections or other trauma. After thestent is fully expanded and cooled, the polymer substantially sets inthe proper apposition, e.g. about a native curvature. Excessive recoilof the stent to a linear configuration is avoided, reducing trauma aboutthe vessel. At the same time, the polymer can have some elastomericproperties in the cooled, hardened state so that the stent can flex withnatural vessel motion. After cooling, the stent exhibits sufficientresistance to inward radial force to reduce restenosis due to, e.g.,lumen wall recoil. The polymer has sufficient strength so that the stentwall can be kept relatively thin while resisting restenosis from lumenwall forces.

Suitable polymers include elastomers that are crosslinked, crystalline,or amorphous, e.g. plasticized PVC, e.g., PVC plasticized with amonomeric plasticizer, e.g., a phthalate, or a polymeric plasticizer.The crosslinked and/or crystalline nature is sufficient to resistexcessive creep or stress relaxation when the polymer is heated andexpanded. The polymer can be crosslinked so that it exhibits the desiredelastomeric properties but not crosslinked to the degree that it becomesexcessively brittle. Too little crosslinking does not establishsufficient resistance to flow during heating and expansion to maintainstent geometry. In addition, crosslinking can be adjusted to adjust themelt or glass transition temperature and transition temperature range. Anarrow melt transition range is desirable, e.g. 5° C. or 10° C. or less.Crosslinking can be achieved by application of radiation such as e-beam,UV, gamma, x-ray radiation or by heat-activated chemical crosslinkingtechniques. Chemical crosslinking agents include peroxides, such asbenzoyl peroxide or dicumyl peroxide (DCP), and azo compounds, such as2,2′-azobis(2,4-dimethyl valeronitrile) or2,2′-azobis[N-(2-propenyl)-2-methylpropionamide]. Radiation techniquesprovide the advantage that the polymer typically does not have to besubstantially heated to achieve crosslinking. An intricate aperturepattern provided in a stent precursor tube can be maintained andheat-induced flow of pre-crosslinked polymer can be avoided. For gammaradiation, an exposure of about 50-300, e.g. 250 kilograys typicallyprovides sufficient crosslinking Melting and crystallizationtemperatures are measured using a differential scanning calorimetry.

The polymer can have elastomeric properties in the melted or softenedstate. Elastomeric properties at melt or glass transition can beinvestigated by measuring the modulus of elasticity or storage modulusas a function of temperature and determining the elastomeric nature ofthe material in the desired expansion temperature range. Referring toFIG. 3, a plot of storage modulus as a function of temperature isprovided. Storage modulus decreases as the material is heated. At themelt or glass transition, a plateau “P” is typically consistent with anelastomeric nature. At much higher temperatures, the modulus drops offmore quickly, indicating a material which could flow under pressure. Todetermine storage modulus, a dynamic mechanical analyzer (Perkin Elmer)can be used. Dynamic mechanical analysis was carried out in tensile modeat an operating frequency of 1 Hz, a static force of 10 mN, andoscillation amplitude of 5 μm (approximately 0.1% strain) and anautomatic tension setting of 125%. Temperature ramps were conducted at4° C./minute over the range −100° C. to 100° C.

Chemically crosslinking PCO also has a direct impact on thethermomechanical properties, e.g. modulus versus temperature, throughthe establishment of a permanent network and indirectly throughmorphological transitions. Revealing such effects by the use of DMA,FIG. 3A shows plots of the tensile storage modulus (E′) versustemperature for cured PCOs prepared with varying amounts of DCP. All ofthe PCO samples are characterized by a solid-like storage modulus (about1.7 GPa) for temperatures below T=−70° C. with this modulus value beinginvariant to the crosslinking density. For temperatures above T=−70° C.,the apparent onset of T_(g) in the PCO samples, E′ begins to decreasegradually to a level that is dependent on crosslink density, butspanning 0.05 to 0.5 GPa. The decrease in the modulus with crosslinkingin this temperature region can be understood from the results of the DSCand wide angle x-ray scattering (WAXS), FIG. 3B, that showedcrosslinking reduces the degree of crystallinity of PCO. It is to beexpected that the crystalline phase will function as both the fixingmechanism for shape memory and a means of controlling room temperaturemodulus over a full order of magnitude. For temperatures nearing T=62°C., close to the melting temperature measured by DSC, the storagemodulus of neat PCO begins to decrease sharply to about 2 MPa at thecompletion of melting at 71° C. As found with DSC, this transitiontemperature is observed mechanically to decrease with increasing degreeof crosslinking. For temperatures greater than T_(m), the modulus ofneat PCO, trace (i), continues to decrease to a point where the materialflows like a viscous liquid, not showing a persistent rubbery plateau(FIG. 3). This feature hampers the applicability of neat PCO for use asa shape memory polymer due to an inability to be deformed as a rubberabove T_(m) without rapid stress relaxation. On the other hand, curedPCO, which contains just 1% peroxide, represented by trace (ii), willallow significant shape memory effects owing to its persistent rubberyplateau above 72° C. As the amount of peroxide increases, the rubberyplateau modulus increases, allowing for enhanced mechanical energystorage, but the transition temperature and the steepness of thetransition decrease. In the case of PCO with 10% DCP, shown as trace (v)in FIG. 3A, the thermomechanical response that is observed isinconducive to shape memory effects as the fixing (crystallization)temperature is lower than room temperature so that shape fixing wouldrequire subambient cooling and the temporary shape would be expected todrift via partial melting. In addition, the melting transition is toobroad for dramatic strain recovery to be expected.

Suitable polymers include elastomers that are typically crosslinkedand/or crystalline and exhibit melt or glass transitions at temperaturesthat are above body temperature and safe for use in the body, e.g. atabout 40 to 50° C. Suitable polymers can have an elastic modulus ofabout 60,000 or 70,000 psi or more at 25° C. (ASTM D638M). Such polymersmay have a variety of room temperature moduli, from rigid glassymaterials having storage moduli of several GPa to compliant rubbers withmoduli as low as tens of MPa. Moreover, the moduli may tuned over therange 0.5<E<10 MPa, as dictated by the end application. Suitablepolymers include polynorbornene, polycaprolactone, polyenes, nylons,polycyclooctene (PCO), blends of PCO and styrene-butadiene rubber,polyvinyl acetate/polyvinylidinefluoride (PVAc/PVDF), blends ofPVAc/PVDF/polymethylmethacrylate (PMMA), polyurethanes,styrene-butadiene copolymers, polyethylene (particularly, crosslinkedpolyethylene), trans-isoprene, block copolymers of polyethyleneterephthalate (PET), blends of polycaprolactone and n-butylacrylate, andPVC, e.g., plasticized PVC, e.g., PVC plasticized with a monomericplasticizer, e.g., a phthalate, or a polymeric plasticizer. A suitablePVAc/PVDF tube is formed by compounding 60-80 parts (by weight) PVAc(B-100, mw 500,000, ChemPoint, Cleveland, Ohio) with 40 to 20 parts PVDF(Grade 1010, Solvay Fluoropolymers, Houston, Tex.). The PVAc/PVDF is acrystalline material that can be utilized with or without crosslinking.

The polymer body can be made of mixtures of polymers or multiple polymerlayers. The polymer forming the stent body can be compounded to includea drug, which elutes from the polymer, or radiopaque material. Thestructural polymer body can be coated with other polymers to carry drugor control drug delivery from the structural polymer. The polymer bodycan also exhibit shape memory properties. This feature of the polymercan be used in combination with the expansion properties discussedabove. For example, the polymer can be configured to remember anenlarged or reduced diameter configuration. For example, the stent canbe delivered into the body, and expanded by a combination of heat andradial pressure as described above. After a time, the stent can beretrieved by reheating the stent. In this case, the heating causes thestent to revert its small diameter condition. The remembered stentdiameter is less than the vessel diameter and the stent can be moreeasily removed from the vessel. Such an application might be useful, forexample, for a stent delivered into the prostate where removal andreplacement is desirable.

Referring to FIGS. 4 and 5, manufacture and use of the stent isillustrated. Referring particularly to FIG. 4, in a first step, apolymer tube 40 is constructed by extrusion or molding a suitablepolymer to an initial diameter di which in the same or greater than thetarget lumen diameter. (For stents made of polymer strands, the strandscan be formed by extrusion, followed by arranging the strands into atube, e.g. by weaving or knitting.) The tube wall is then cut to providea pattern of open areas in a desirable geometric pattern, e.g. by lasercutting. The polymer can be recrystallized or crosslinked, if necessary.Next, the tube is heated typically near or above the melt or glasstransition and mechanically deformed to a small diameter, suitable fordelivery. The tube is cooled, e.g. to room temperature. The tube isassembled onto a catheter, delivered into the body, and expanded byapplication of heat, to the melt or glass transition, while inflatingthe balloon as discussed above. (If the polymer has shape memoryproperties, the polymer tends to expand upon heating to a larger,remembered diameter.

Referring particularly to FIG. 5, a polymer tube 50 is constructed to aninitial diameter di, smaller than the vessel diameter. The tube wall iscut to provide an open area in a desirable pattern so that when thestent is expanded, a compression-resistant geometry will result. Thepolymer is recrystallized or crosslinked, if necessary. The tube isassembled in a catheter, delivered into the body, and expanded byheating to the melt or glass transition temperature while inflating theballoon to provide an outward radial expansion, as discussed above. Ifthe polymer has shape memory properties, the stent can be subsequentlyre-heated so it reverts back to its remembered small diameterconfiguration and removed from the body.

In particular embodiments, the stent can have an expanded inner diameterof about 2 to about 20 mm. The initial inner diameter can be in therange of about 1 to about 3 mm. The wall thickness is in the range ofabout 0.005 mm to 20 mm. The wall thickness variation between deliveryand expanded states is within about 10%, 5% or 1% or less. The ratio ofthe open area to the wall area in the expanded state is about 0.5 to 0.7or more. (The tube wall area is the area of the tube defined by polymer.The open area is the open area defined by the apertures.) A particularstent for coronary use has an initial diameter of about 2 mm, anexpanded diameter of about 4 mm, and a wall thickness of about 0.005 mmto 0.1 mm. The stent can be used for various applications includingcoronary, neuro, carotid, peripheral vasculature, ureteral, and prostatelumens. The stent is particularly useful where the lumen path is highlycurved or irregular.

The catheter can be, e.g. an angioplasty balloon catheter with arelatively non-distendable inflating balloon suitable for expansion ofoccluded regions of a vascular lumen. The balloon may include a polymersuch as PET, which has a burst pressure of about 1.5 to 5 atm or more.The stent can be heated by heating the balloon inflation fluid. Theballoon inflation fluid can be heated by e.g., heating the fluiddelivery device outside the body using e.g., resistive heating tape.Alternatively, the catheter can carry a heating device. For example, aresistive heater or RF heater can be provided in the interior of theballoon. A heated balloon catheter is described in Abele et al. U.S.Pat. No. 5,496,311 and U.S. Pat. No. 4,955,377. Vessel heating asdescribed in the '311 can be used in combination with stent delivery asdiscussed herein. Alternatively, the stent can be heated directly. Forexample, the polymer can be compounded to include a material, such asmagnetic particles, which are susceptible to heating by magneticeffects, such as hysteresis effects. A magnetic field can be imposed onthe stent body by a source on a catheter or outside the body. Particlesare available as the Smartbond System from Triton Systems, Inc.,Chelmsford, Mass. Heating by magnetic effects is discussed in U.S. Pat.No. 6,056,844. The stent can also be heated during delivery withoutapplying expansion force to soften the stent, improving its flexibilityand thus improving delivery to a treatment site through a tortuousvessel path.

EXAMPLE

A polycyclooctene polymer (Vistenemer 8012 pellets, mw 90,000, Degussa,N.J.) is melt processed in an extruder to produce a tube havingdimensions of about 0.118 inch O.D. and 0.070 inch I.D. (wall thicknessabout 0.024 inch). The tube is cut to a length of about 4 cm. The tubeis subject to UV excimer laser ablation cutting to provide an aperturepattern of rectangular slots having a width of about 0.2 mm and a lengthof about 8 mm. Beam energy and pulse rate are selected to avoidsubstantial heating or melting of the polymer. The polymer can becompounded with about 10% TiO₂ (T8141, Dupont) to enhance absorption oflaser radiation. A suitable pattern is consistent with the Express stent(commercially available from Boston Scientific, Natick, Mass.).(Alternatively, a pattern as described in Palmaz U.S. Pat. No. 4,733,665can be used.) The tube is heated to a temperature below its melt point,e.g., to about 39 to 40° C. in a water bath and expanded by ballooncatheter to a diameter of about 5 mm and positioned on a mandrel (PTFEtube) to maintain the expanded shape and diameter. The tube is thencooled to room temperature. The polymer is then crosslinked by e-beamradiation at 250 K Grays (Steris Isomedics Services, Northborough,Mass.). Crosslinking fixes the stent in the condition. (The crosslinkedPCO has an elastic (Youngs) modulus of about 74945 psi at about 25° C.(ASTM D638M)). The stent is heated to the polymer melt temperature,about 45° C. and collapsed over a deflated balloon (diameter of about 2mm) with a 4 mm inflated maximum diameter and 2 cm length. (A suitableballoon catheter is a 75 cm Meditech UltraThin Catheter, available fromBoston Scientific, Natick, Mass.). The balloon and stent are immersed ina water bath of about 42 to 45° C. and water the same temperature isused to inflate the balloon. The stent is expanded to about 4 mmdiameter (ID) at an inflation pressure of about 1 to 1.5 atm (measuredat the delivery syringe). After expansion, the heating is discontinuedand the balloon and inflation fluid allowed to cool to body temperature(while the balloon remains inflated). Alternatively, a cooled contrastfluid can be circulated to the balloon. The stent exhibits no visiblereduction in wall thickness or irregular flow of polymer into the stentopen areas. In addition, in the heated, expanded state, the stent can bebent around a mandrel of about 0.75 cm radius without kinking. After thestent is cooled, it maintains the curved form.

Referring to FIGS. 6-8, stent 100 includes a coiled rod 120 composed ofa polymer. Stent 100 includes an elongated portion 140 having a generaldiameter d₁ and an end portion 160 having a maximum diameter d₂. Generaldiameter d₁, for example, may be between about 3 mm and about 25 mm,more preferably between about 6 mm and about 14 mm, and maximum diameterd₂, for example, may be between about 7 mm and about 30 mm, morepreferably between about 10 mm and about 17 mm. When stent 100 isdesigned, for example, for insertion into a urethra, the stent may havean overall length, for example, of between about 3 mm and about 15 mm,preferably between about 6 mm and 10 mm, and end portion 160 may have alength, for example, of from about 3 mm to about 15 mm, preferably fromabout 5 mm to about 10 mm. End portion 160 is in a flared position.Referring to FIG. 8, stent 100 is shown with end portion 160 in acollapsed position that can be reverted with heating to the expandedposition shown in FIGS. 6 and 7.

Generally, the portion of the stent in the collapsed position that canbe reverted to the expanded position is, for example, greater than 5%,10%, or even 25% of the overall length L of the stent, and less than 80%or 65% of the overall length L of the stent. For example, the of theoverall length L of the stent may be between 10% and 65% of the overalllength L of the stent.

The polymers preferably are cross-linked and/or crystalline elastomersthat have melt or glass transition temperatures that are above bodytemperature, for example, greater than 45° C. or 55° C. The degree ofcross-linking can be used to adjust, for example, the melt or glasstransition temperature, and range, of the polymer. The polymerpreferably has a relatively narrow, for example, less that 5° C. or 10°C., melt or glass transition temperature range.

The polymer preferably has elastomer properties in its melted orsoftened state. Preferred polymers have an elastic modulus, for example,of about 60,000 psi or 70,000 psi or more at 25° C. (ASTM D638).

Examples of polymers include polynorbornene and copolymers ofpolynorbornene, blends of polynorbornene with KRATON® (thermoplasticelastomer) and polyethylene, styrenic block copolymer elastomers (e.g.,styrene-butadiene), polymethylmethacrylate (PMMA), polyethylene,polyurethane, polyisoprene, polycaprolactone and copolymers ofpolycaprolactone, polylactic acid (PLA) and copolymers of polylacticacid, polyglycolic acid (PGA) and copolymers of polyglycolic acid,copolymers of PLA and PGA, polyenes, nylons, polycyclooctene (PCO),polyvinyl acetate (PVAc), polyvinylidene fluoride (PVDF), blends ofpolyvinyl acetate/polyvinylidine fluoride (PVAc/PVDF), blends ofpolymethylmethacrylate/polyvinyl acetate/polyvinylidine fluoride(PVAc/PVDF/PMMA) and polyvinylchloride (PVC).

In some embodiments, the polymers above are also useful for the stentsof FIGS. 1, 4 and 5.

Particular polyurethanes are made by reacting (A) a polyol, (B) a chainextender dihydroxyl-terminated POSS and (C) a diisocyanate, where POSSstands for a polyhedral oligomeric silsesquioxane diol. The polyol (A)can be polyethylene glycol (PEG), polycaprolactone (PCL),polycyclooctene (PCO), trans-1,4 butadiene, transisoprene,polynorbornene diol and polymethacrylate copolymer, the chain extender(B) can be TMP cyclopentyldiol-POSS, TMP cyclohexyldiol-POSS, TMPisobutyldiol-POSS, trans-cyclohexanediolcyclohexane-POSS, ortranscyclohexanediolisobutyl-POSS and the diisocyanate (C) can beselected from a large number of diisocyanates and is preferably 4,4′diphenyl methylene diisocyanate (MDI). Other diisocyanates (C) that aresuitable for use in the synthesis of hybrid polyurethane SMPs include:toluene-2,4-diisocyanate (TDI), toluene-2,6diisocyanate,hexamethylene-1,6-diisocyanate (HDI), 4,4′ diphenylmethane diisocyanate(MDI), isophorone diisocyanate (IPDI), and hydrogenate4,4′-diphenylmethane diisocyanate (H12MDI).

The particular polyurethanes described directly above may be preparedthe non-limiting schemes illustrated below. A graph of heat flow as afunction of temperature for several POSS polyurethanes is shown in FIG.8A and a graph of storage modulus as a function of temperature forseveral POSS polyurethanes is shown in FIG. 8B.

This scheme shows an example of synthesis of TPU using polyethyleneglycol as polyol, TMP Isobutyldiol-POSS as chain extender to react with4,4′ diphenyl methylene diisocyanate in toluene.

This scheme shows an example of synthesis of TPU using polycaprolactonediol as polyol, TMP Isobutyldiol-POSS as chain extender to react with4,4′ diphenyl methylene diisocyanate.

This scheme shows an example of synthesis of TPU using polyocycloocteneas polyol, TMP Isobutyldiol-POSS as chain extender to react with 4,4′diphenyl methylene diisocyanate.

Any of the polymers mentioned above may be filled with, for example,nanoparticles of clay and silica to, for example, increase the modulusof the plastic. Dispersing agents and/or compatibilizing agents may beused, for example, to improve the blending of polymers and the blendingof polymers with fillers. Dispersing agents and/or compatibilizingagents include, for example, ACRAWAX® (ethylene bis-stearamide),polyurethanes and ELVALOY® (acrylic-functionalized polyethylene). Thepolymers can be cross-linked by application of radiation such as e-beam,UV, gamma, x-ray radiation or by heat-activated chemical crosslinkingtechniques. Radiation techniques provide the advantage that the polymertypically does not have to be substantially heated to achievecrosslinking. For e-beam radiation, an exposure of about 200-300, e.g.250 kilograys, typically provides sufficient crosslinking.

Referring to FIG. 9, wrapping fixture 200 can be used for making coiledstent 100. Wrapping fixture 200 includes a base 220 for support, amandrel 240 with a flared end 260, slits 280 for fixing the plastic rod120, an aperture 290 for fixing the plastic rod 120 on the non-flaredend and a fixing screw 299 for releasably fixing mandrel 240 to wrappingfixture 200.

Stent 100 is manufactured from plastic rod 120 made by a variety ofmethods known in the art (e.g., extrusion, coextrusion, injectionmolding, casting, compression molding). If casting is used, the polymersmay have tunable critical temperatures and rubber modulus have beensynthesized using a thermosetting random copolymer formed from two vinylmonomers that yield controlled T_(g) and casting-type processing. Suchcopolymers were crosslinked with a difunctional vinyl monomer(crosslinker), the concentration of crosslinker controlling the rubbermodulus and thus the work potential during recovery. Rod 120 may have adiameter, for example, of about 0.25 mm to about 2.5 mm or more. Plasticrod 120 and wrapping fixture 200 are heated to the softening temperatureof the polymer, making the plastic rod 120 malleable. The rod 120 isinserted into aperture 290 that is machined through mandrel 240 to fixthe rod at the starting end. The rod 120 is tightly wrapped aroundmandrel 240, including flared end 260. To fix plastic rod 120 in place,plastic rod 120 is pushed into slits 280. The overall length of thestent 100 may be, for example, about 2 mm to about 150 mm or more. Theoverall length required depends upon the application. The plastic rod120, now fixed in place on the mandrel 240, is heated to above thesoftening point of the material and maintained at that temperature longenough to anneal the rod and fix the shape. Typically, the time requiredto fix the shape is from about 0.25 hr to 10 hr or more. After cooling,stent 100 is removed from mandrel 240. Before packaging, the flared endof the coil is tapered down and collapsed so that the diameter along theentire length of the stent is approximately d₁. Collapsing the flaredend 160 of stent 100 allows for ease of insertion, for example, into arestricted prostatic urethra.

EXAMPLE 1

A 56:24:20 mixture of PVAc/PVDF/PMMA is dry bended and loaded into thehopper of an extruder. The PVAc is grade B-100, the PVDF is SolvaySOLEF® 1010 and the PMMA is Atofina PLEXIGLAS® V045. The mixture is meltprocessed to produce 1.27 mm (0.05 inch) monofilament. The rod is madeinto a coil by winding it around wrapping fixture 200. The fixture andthe rod are immersed into a 50° C. water bath. At this temperature, therod becomes malleable enough to wind easily around the mandrel andsecured in place to prohibit the uncoiling of the helical shape. Themandrel is removed from the fixture with the stent locked in place andplaced into an oven at 110° C. for one hour to anneal the stent. Thisannealing process locks the permanent shape of the coil. The mandrel andcoil are cooled to room temperature, and the stent is removed from themandrel. The stent had on overall length of approximately 73 mm and aflared end portion length of approximately 7 mm. The diameter d₁ of thebody is approximately 6 mm and the maximum diameter d₂ of the flare onthe open end is approximately 11 mm. Before packaging, the flared end ofthe coil is tapered down with brief heating to 50° C. and manipulation,followed by cooling, so that the diameter is approximately 6 mm alongthe entire length of the stent.

Example 2

A 70:30 mixture of PVAc/PVDF is dry bended and loaded into the hopper ofan extruder.

The mixture is melt processed to produce 1.27 mm (0.05 inch)monofilament. The rod is made into a coil by winding it around wrappingfixture 200. The fixture and the rod are immersed into a 50° C. waterbath. At this temperature, the rod becomes malleable enough to windeasily around the mandrel and secured in place to prohibit the uncoilingof the helical shape. The mandrel is removed from the fixture with thestent locked in place and placed into an oven at 110° C. for one hour toanneal the stent. This annealing process locks the permanent shape ofthe coil. The mandrel and coil are cooled to room temperature, and thestent is removed from the mandrel. The stent had on overall length ofapproximately 73 mm and a flared end portion length of approximately 7mm. The diameter d₁ of the body is approximately 6 mm and the maximumdiameter d₂ of the flare on the open end is approximately 11 mm. Beforepackaging, the flared end of the coil is tapered down with brief heatingto 50° C. and manipulation, followed by cooling, so that the diameter isapproximately 6 mm along the entire length of the stent.

Referring to FIG. 10-12, stent 100 may be, for example, inserted intorestricted urethra 300 on delivery tube 320. During insertion, endportion 160 is in a collapsed position. After insertion, warm water(e.g., 45° C.-55° C.) is flushed through delivery tube 320 that is inthermal contact with stent 100. Heating reverts the collapsed end 160 toa flared, expanded position (FIG. 12). The flared, expanded positionallows stent 100 to remain fixed in position, for example, in theprostatic urethra.

Referring to FIG. 13 for a little more detail, delivery tube 320 is along cylindrical tube into which a ureteral scope 380 is inserted.Delivery tube 320 has a distal end 330 over which stent 100 is placed.Delivery 320 is fitted with a side port 350 including a stopcock 370through which saline can be flushed for irrigation. Delivery tube 320with stent 100 in place is delivered into, for example, the prostaticurethra with the aid of a ureteral scope. Once stent 100 is in place,hot saline is flushed through port 350 to revert the collapsed end 160to a flared, expanded position (FIG. 12). The flared, expanded endallows stent 100 to remain fixed in position, for example, in oradjacent the prostatic urethra or external sphincter between theprostate and the bladder to prevent migration. The direction of theflare can, of course, be oriented in other directions. The scope anddelivery tube 320 are withdrawn, leaving stent 100 in place.

Referring to FIGS. 13-13B, an alternative delivery system is illustratedthat includes a tube 320 with a screw on tip 331 onto which a stent 100is placed after collapsing the end portion 160. The assembly is insertedinto, for example, the prostatic urethra.

Other delivery methods are within the claims. Stent 100 may be, forexample, inserted into restricted urethra 300 on balloon catheter (notshown). During insertion, end portion 160 is in a collapsed position.After insertion, warm water is flushed through the guide wire lumen ofthe balloon catheter to flood the area and to heat the stent. Heating ofthe stent by the water reverts the collapsed end 160 to a flared,expanded position The flared, expanded position allows stent 100 toremain fixed in position, for example, in the prostatic urethra. Ifthere is an obstruction in the lumen into which the stent is deployedthe stricture can be dilated using the balloon to help the stent openfully and maintain a uniform diameter inside the vessel.

Referring to FIG. 14, coiled stent 100 with an end 160 in the expandedposition can be removed with the aid of a catheter equipped with agrasping device 360 and a ureteral scope 380 for visualizing stent 100in, for example, the prostatic urethra. Once end 400 of stent 100 hasbeen visualized with ureteral scope 380, the stent is grasped withgrasping device 360. Next, ureteral scope 380 is removed from thecatheter and is replaced with a heating device (not shown), e.g. acatheter. Heating stent 100 above the softening point of the polymer,e.g., from about 45° C. to about 55° C. for polycyclooctene (PCO), andpulling the end 400 of stent 100 through the orifice 420 allows thestent to be removed in a substantially uncoiled state.

Although FIG. 11 shows heating of stent 100 with a warm liquid on adelivery tube, other heating methods are possible. For example, heatingmay be accomplished with the use of IR, RF or inductive heating.

Although insertion into a prostatic urethra has been used as an example,insertion of stent 10 into other body lumens or cavities is possible.For example, other body lumens or cavities include the biliary duct,cystic duct, a ureter, a bulbar urethra or a hepatic duct.

Referring to FIG. 15, the modulus of polycyclooctene (PCO) that can, forexample, be the polymer of stent 100 is shown as a function oftemperature. Below approximately −65° C. (T_(g), region A), PCO exists arigid, glassy polymer. Above T_(g), but below T_(m), PCO exists as aflexible elastomer (region B). Above T_(m), PCO exists as a relativelylow modulus elastomer. Above T_(m), for example, stent 100 composed ofPCO can be removed from a lumen or cavity of the body, the prostaticurethra, for example, in a substantially uncoiled state.

Referring to FIG. 16, the modulus of a ternary blend of PVAc/PVDF/PMMAthat can, for example, be the polymer of stent 100 is shown as afunction of temperature. Adding PMMA offers the advantage, for example,of increasing the modulus of the blend.

Referring once again to FIG. 15 and to FIGS. 6-8, heating stent 100 inthe expanded position briefly above T_(m) (region C) and then coolingrapidly below T_(m) (e.g., region B) “freezes” stress into stent 100.Stent 100 reverts from its collapsed position to its expanded positionupon re-heating above T_(m) (region C) because the modulus of thematerial lowers sufficiently to release the residual stress that was“frozen” into stent 100 during the rapid cooling.

FIGS. 17-28 show other examples of stents.

Referring to FIGS. 17 and 18, coiled stent 500 has two end portions 520and 540 in expanded positions. End portions 520 and 540 can be collapsed(FIG. 18) for ease of insertion into a body cavity or lumen, andreverted with heat to expanded positions.

Referring to FIG. 19, coiled stent 600 has two end portions 620 and 660and a central portion 640 in expanded positions. All three portions maybe collapsed (not shown), and then reverted to expanded positions.

Referring to FIGS. 20 and 21, coiled stent 700 made from a flattenedtube 710 has an end portion 720 in an expanded position. The flattenedtube can, for example, add strength to the stent. The tube can have amajor diameter, for example, of between about 1.0 mm to about 3.0 mm,more preferably between about 1.5 mm to about 2.25 mm, and major innerdiameter, for example, of between about 0.5 mm to about 2.5 mm, morepreferably between about 1.25 mm and about 1.75 mm. End portion 720 maybe collapsed (not shown) and then reverted to an expanded position.Referring to FIG. 21, flattened tube 710 has an interior 740 that may befilled with, for example, a medicament. The medicament, for example, maybe triclosan or salicylic acid. Release of medicament from flattenedtube 710, for example, may reduce the risk of infection. Interior 740may also be filled with, for example, paclitaxel or mitoxantrone.Release of the these medicaments from interior 740 may be, for example,useful for treating prostate cancer and reducing prostatic hyperplasia.

In general, any filler, e.g., a therapeutic agent, can be used to fillinterior 740. A therapeutic agent can be a genetic therapeutic agent, anon-genetic therapeutic agent, or cells. Therapeutic agents can be usedsinglularly, or in combination. Therapeutic agents can be, for example,nonionic, or they may be anionic and/or cationic in nature.

Exemplary non-genetic therapeutic agents include: (a) anti-thromboticagents such as heparin, heparin derivatives, urokinase, and PPack(dextrophenylalanine proline arginine chloromethylketone); (b)anti-inflammatory agents such as dexamethasone, prednisolone,corticosterone, budesonide, estrogen, sulfasalazine and mesalamine; (c)anti-neoplastic/antiproliferative/anti-miotic agents such as paclitaxel,5-fluorouracil, cisplatin, vinblastine, vincristine, epothilones,endostatin, angiostatin, angiopeptin, monoclonal antibodies capable ofblocking smooth muscle cell proliferation, and thymidine kinaseinhibitors; (d) anesthetic agents such as lidocaine, bupivacaine andropivacaine; (e) anti-coagulants such as D-Phe-Pro-Arg chloromethylketone, an RGD peptide-containing compound, heparin, hirudin,antithrombin compounds, platelet receptor antagonists, anti-thrombinantibodies, anti-platelet receptor antibodies, aspirin, prostaglandininhibitors, platelet inhibitors and tick antiplatelet peptides; (f)vascular cell growth promoters such as growth factors, transcriptionalactivators, and translational promotors; (g) vascular cell growthinhibitors such as growth factor inhibitors, growth factor receptorantagonists, transcriptional repressors, translational repressors,replication inhibitors, inhibitory antibodies, antibodies directedagainst growth factors, bifunctional molecules consisting of a growthfactor and a cytotoxin, bifunctional molecules consisting of an antibodyand a cytotoxin; (h) protein kinase and tyrosine kinase inhibitors(e.g., tyrphostins, genistein, quinoxalines); (i) prostacyclin analogs;(j) cholesterol-lowering agents; (k) angiopoietins; (l) antimicrobialagents such as triclosan, cephalosporins, aminoglycosides andnitrofurantoin; (m) cytotoxic agents, cytostatic agents and cellproliferation affectors; (n) vasodilating agents; (o) agents thatinterfere with endogenous vasoactive mechanisms; (p) inhibitors ofleukocyte recruitment, such as monoclonal antibodies; (q) cytokines, and(r) hormones.

Exemplary genetic therapeutic agents include anti-sense DNA and RNA aswell as DNA coding for: (a) anti-sense RNA, (b) tRNA or rRNA to replacedefective or deficient endogenous molecules, (c) angiogenic factorsincluding growth factors such as acidic and basic fibroblast growthfactors, vascular endothelial growth factor, epidermal growth factor,transforming growth factor α and β, platelet-derived endothelial growthfactor, platelet-derived growth factor, tumor necrosis factor α,hepatocyte growth factor and insulin-like growth factor, (d) cell cycleinhibitors including CD inhibitors, and (e) thymidine kinase (“TK”) andother agents useful for interfering with cell proliferation. Also ofinterest is DNA encoding for the family of bone morphogenic proteins(“BMP's”), including BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 (Vgr-1), BMP-7(OP-1), BMP-8, BMP-9, BMP-10, BMP-11, BMP-12, BMP-13, BMP-14, BMP-15,and BMP-16. Currently preferred BMP's are any of BMP-2, BMP-3, BMP-4,BMP-5, BMP-6 and BMP-7. These dimeric proteins can be provided ashomodimers, heterodimers, or combinations thereof, alone or togetherwith other molecules. Alternatively, or in addition, molecules capableof inducing an upstream or downstream effect of a BMP can be provided.Such molecules include any of the “hedgehog” proteins, or the DNA'sencoding them.

Vectors for delivery of genetic therapeutic agents include viral vectorssuch as adenoviruses, gutted adenoviruses, adeno-associated virus,retroviruses, alpha virus (Semliki Forest, Sindbis, etc.), lentiviruses,herpes simplex virus, replication competent viruses (e.g., ONYX-015) andhybrid vectors; and non-viral vectors such as artificial chromosomes andmini-chromosomes, plasmid DNA vectors (e.g., pCOR), cationic polymers(e.g., polyethyleneimine, polyethyleneimine (PEI)), graft copolymers(e.g., polyether-PEI and polyethylene oxide-PEI), neutral polymers PVP,SP1017 (SUPRATEK), lipids such as cationic lipids, liposomes,lipoplexes, nanoparticles, or microparticles, with and without targetingsequences such as the protein transduction domain (PTD).

Cells for use include cells of human origin (autologous or allogeneic),including whole bone marrow, bone marrow derived mono-nuclear cells,progenitor cells (e.g., endothelial progenitor cells), stem cells (e.g.,mesenchymal, hematopoietic, neuronal), pluripotent stem cells,fibroblasts, myoblasts, satellite cells, pericytes, cardiomyocytes,skeletal myocytes or macrophage, or from an animal, bacterial or fungalsource (xenogeneic), which can be genetically engineered, if desired, todeliver proteins of interest.

Referring to FIG. 22, coiled stent 800 made from tape 810 has an endportion 820 that may be collapsed and then reverted to an expandedposition. Tape 810, for example, may have a thickness from about 0.5 mmto about 2.0 mm, more preferably from about 0.75 mm to about 1.25 mm,and a width, for example, of from bout 1.0 mm to about 3.0 mm, morepreferably from about 1.75 mm to about 3.00 mm. In this particularembodiment, an aperture 840 is provided so that a trailing string (notshown) can be included for ease of removal with the grasping deviceshown in FIG. 14.

Referring to FIGS. 23-25, coiled stent 900 is made from plastic rod andhas an end portion 920 that may be collapsed and then reverted to anexpanded position. Stent 900 includes a plurality of protruding elements940 that are integral with and extend outwardly from the plastic rodfrom which the stent is made. Referring now to FIGS. 26-28, coiled stent1000 is made from plastic rod, for example, oriented monofilament, andhas an end portion 1020 that may be collapsed and then reverted to anexpanded position. Stent 1000 includes a plurality of protrudingelements 1060 that extend outwardly from the plastic rod from which thestent is made. The friction provided by protruding elements 940,1060 canhelp to hold stent 900,1000 in place within, for example, the prostaticurethra.

Protruding elements 940 are made, for example, by cutting into theplastic rod with, for example, a sharp edged instrument, for example, aknife, so as penetrate a depth into the plastic rod. The depth ofpenetration P is adjusted to provide acceptable frictional properties,while minimizing the impact on the mechanical properties of the plasticrod. In some implementations, the maximum depth of penetration into theplastic rod, measured inwardly from the outer surface of the plastic rodis, for example, from about 1 to about 50% of the average thickness ofthe plastic rod. If the depth penetration is too large, the mechanicalproperties of the plastic rod may be reduced and if the depth ofpenetration is too low, the resulting protruding elements may be toosmall to provide the appropriate frictional properties when expanded ina body cavity or lumen, for example, the prostatic urethra. Othercutting means are possible, for example, water knife and laser cuttingmeans, to reduce the impact of the cutting on the mechanical propertiesof the plastic rod. The shape of the plastic rod from which the stent ismade may be of other forms than that shown above. For example, it may bein the form of, for example, a coiled elongated flattened tube and theflattened tube may include a central opening that includes a medicamentthat can be released by the inserted stent.

In some implementations, stent 900 is manufactured from plastic rod madeby a variety of methods known in the art (e.g., extrusion andcoextrusion). The plastic rod may have a diameter, for example, of about0.25 mm to about 2.5 mm or more. In a preferred method, the protrudingelements are put onto the plastic rod before wrapping the mandrel shownand discussed above. After wrapping the mandrel, the plastic rod andwrapping fixture 200 are heated to the softening temperature of thepolymer, making the plastic rod malleable. The protrusions are annealedin the “up” position, that is, with the protruding elements extendingoutwardly by “prying up” the protruding elements that results fromcutting. Prying up the protruding elements may be achieved by, forexample, running a surface across the protruding elements in a directionopposite the cut direction. Annealing is continued to fix the shape.After cooling, the stent is removed from mandrel. Before packaging, theflared end of the coil is tapered down along with protruding elementsand collapsed so that the diameter along the entire length of the stentis approximately d₁. Collapsing the flared end and protruding elementsallows for ease of insertion, for example, into a restricted prostaticurethra. In some implementations, the flared end and the main body arecollapsed to have a diameter less than d₁. Upon heating, the end portionreverts to its expanded position and the protruding elements revert totheir up positions. If a medicament is placed in the cavities 960 fromwhich the protruding elements 940 are carved, it may be released uponexpansion of the stent.

Referring to FIGS. 26-28, stent 1000 with protruding elements 1060 ismade by, for example, wrapping a thicker plastic rod with a thinnerplastic rod, for example, a monofilament, that includes a plurality ofconstrictions, for example, knots along its length. The elevation Eabove an outer surface of the thicker plastic rod is adjusted to provideacceptable frictional properties. In some implementations, the maximumelevation above an outer surface of the thicker plastic rod is, forexample, from about 1 to about 50% of the average thickness of thickerplastic rod. If the elevation E is too large, insertion of stent 1000into, for example, a prostatic urethra may become difficult and if themaximum elevation above the protruding elements is too small, theprotruding elements may not provide the appropriate frictionalproperties when expanded in a body cavity or lumen, for example, theprostatic urethra. The shape of the rods from which the stent is mademay be of other forms than that shown above. For example, it may be inthe form of, for example, a coiled elongated flattened tube and theflattened tube may include a central opening that includes a medicamentthat can be released by the inserted stent.

In some implementations, stent 1000 is manufactured from plastic rodmade by a variety of methods known in the art (e.g., extrusion andcoextrusion). The thicker plastic rod may have a diameter, for example,of about 0.25 mm to about 2.5 mm or more. The thinner plastic rod fromwhich the protruding elements are fashioned may have a diameter of, forexample, from about 0.2 mm to about 20 mm. In a preferred method, theconstrictions, for example, knots, are placed on the thinner plastic rodand the thinner plastic rod is wrapped around the outer surface of thethicker plastic rod. The ends of the thinner plastic rod are heat stakedto hold the thinner plastic rod onto the outer surface of the thickerplastic rod. Now, the assembly of the thinner and thicker plastic rod iswrapped around the mandrel shown and discussed above. After wrapping themandrel, the plastic rods and wrapping fixture 200 are heated to thesoftening temperature of the polymer of the thicker plastic rod, makingthe plastic rod malleable Annealing is continued to fix the shape. Aftercooling, the stent is removed from mandrel. Before packaging, the flaredend of the coil is tapered down so that the diameter along the entirelength of the stent is approximately d₁. Collapsing the flared end andprotruding elements allows for ease of insertion, for example, into arestricted prostatic urethra. In some implementations, the thinnerplastic rod may contain a medicament that is released upon expansion,for example, in the prostatic urethra. In other implementations, thethinner plastic rod is made of a degradable material and the degradablematerial is filled with a medicament.

In some embodiments, the entire stent, for example, the stents of FIGS.4, 5, 20, 23 and 26, may have an expanded position.

Referring to FIGS. 29 and 30, an implantable stent includes a tubularmember 1102 that is formed from a polymeric material, e.g., PCO. Tubularmember 1102 includes a wall with thickness W₁ and has a first transversedimension OD₁, and a first longitudinal length L₁ that is measured atfirst transverse dimension. Tubular member 1102 is sized for deliveryinto a lumen. Upon exposure to an elevated temperature, e.g., 40, 50, or60° C., and to outward mechanical forces, e.g., delivered by a balloon,tubular member 1102 can be expanded (FIG. 30) to a second transversedimension OD₂ that is, e.g., about fifty percent larger than the firsttransverse dimension OD₁ within the lumen. First and second transversedimensions are measured from an outer surface 1106, 1108 of the tubularstructure in its unexpanded and expanded state, respectively. In itsexpanded state, the tubular member has a wall with thickness W₂ and asecond longitudinal length L₂, measured when at the second transversedimension. After expansion from the first OD₁ to the second OD₂transverse dimension, the second longitudinal length L₂ decreases byless than about fifty percent, measured relative to the firstlongitudinal length L₁. This reduced forshortening can improve, forexample, placement accuracy within the lumen.

FIGS. 31 and 32 show an embodiment employing an elongated tubular member1110 that can be expanded in a manner similar to that described above inreference to FIGS. 29 and 30. FIGS. 31 and 32 illustrate that stentshaving reduced foreshortening can be configured to have a variety ofdimensions to enable the stents to be used in a variety of lumens withinthe body. Referring particularly to FIG. 31, the unexpanded tubularstructure has an unexpanded transverse dimension OD′₁, an unexpandedlength L′₁ and an unexpanded wall thickness W′₁. Referring particularlyto FIG. 32, the expanded tubular member has an expanded transversedimension OD′₂, an expanded length L′₂ and an expanded wall thicknessW′₂.

In some embodiments, the wall thickness of the tubular member decreasesby greater than about twenty percent, e.g., greater than about fiftypercent, greater than about fifty-five percent, greater than about sixtypercent, greater than about sixty-five percent, greater than aboutseventy-five percent, or more, e.g., greater than about ninety percent,after expansion from the first transverse dimension to the secondtransverse dimension. Without wishing to be bound by any particulartheory, it is believed that a relatively large decrease in the wallthickness in going from the unexpanded state to the expanded state atleast partially explains the observed reduced foreshortening.

In specific embodiments, after expansion from the first transversedimension to the second transverse dimension that is at least aboutseventy-five percent larger than the first transverse dimension, thesecond longitudinal length decreases by less than about thirty percent,e.g., less than twenty-five percent, less than twenty percent, less thanfifteen percent, or less than ten percent, measured relative to thefirst longitudinal length.

In a specific implementation, the tubular member is approximatelycircular in transverse cross-section. In other embodiments, the tubularmember has other shapes in transverse cross-section. For example, thetubular member can be square, rectangular, pentagonal, hexagonal,octagonal, or elliptical in transverse cross-section.

Generally, the polymeric material has relatively low softeningtemperature so that high temperatures do not need to used within thebody. For example, the polymer can have a softening temperature fromabout 40° C. to about 60° C., e.g., 45, 50, 55, or 58° C.

The polymeric material can be non-cross-linked, cross-linked, shapememory, or non-shape memory. Generally, suitable polymeric materialsinclude those discussed above, e.g., nylons, polyurethanes, or PVAc/PVDFblends, and those discussed below. Specific polymeric materials includepolycyclooctene (PCO), styrenic elastomers, styrenic block copolymers,styrene-butadiene rubber, polyolefins, trans-isoprene, plasticized PVC,e.g., PVC plasticized with a monomeric plasticizer, e.g., a phthalate,or a polymeric plasticizer, or blends of these polymers. In someembodiments, the polymeric material has an elastic modulus of greaterthan about 50,000 psi, e.g., greater than about 75,000, greater thanabout 100,000, greater than about 200,000, greater than about 250,000,or more, e.g., greater than about 500,000 psi. Without wishing to bebound by any particular theory, it is believed that proper selection ofthe polymeric material at least partially explains the observed reducedforeshortening.

The polymeric material can include fillers, e.g., a radio-opaque agent,or a thermal conductor. Examples of radio-opaque materials includebismuth carbonate, barium sulfate, or mixtures of these materials.Examples of thermal conductors include a boron nitride, a metal, ormixtures of these materials.

A stent can have a shape in memory, e.g., a curved shape. For example,the stent can have an unexpanded shape that is substantially straight,and an expanded shape that is curved (FIG. 33). A curved tubular membercan enable better retention in a deployed region of the lumen such thatthe stent has a reduced likelihood for movement within a lumen. Othermemorized shapes are possible. For example, the stent may have a flaredend, or two flared ends after expansion, as shown in FIG. 34. Flaredends can also enable better retention in a deployed region of a lumen.

Referring now to FIGS. 32 and 35, a stent can have a smooth outersurface after expansion, like that of FIG. 32, or the stent can have anouter surface that includes a plurality of protruding elements 1112after expansion, like that of FIG. 35. As was discussed in reference toFIG. 23, the friction provided by the protruding elements can help holdthe stent in place within a lumen.

Referring back to FIG. 31, a tubular member can include apertures 1114defined in a wall, when this is desired. In some embodiments, aperturesare advantageous because they can allow tissue to grow into theapertures, thereby enabling better retention in the lumen.

In some embodiments, the tubular member is delivered to a lumen, e.g., apulmonary lumen, an esophageal lumen, a biliary lumen, an enteral lumen,a ureteral lumen, or a urethral lumen. Delivery to the lumen can be doneon, e.g., a balloon catheter. After expansion of the tubular memberwithin the lumen, the delivery vehicle can be removed, with the stentremaining in place within the lumen of the patient.

In a specific embodiment, an unexpanded tubular stent is cylindrical inshape, has a smooth outer surface, and is made of PCO filled with aboutforty percent by weight of a boron nitride for radio-opacity, and forenhanced thermal conductivity. The stent has an unexpanded wallthickness of about 3 mm, an outer diameter of approximately 10 french,and an unexpanded length of approximately 25 mm. After expansion on aheated balloon at 50° C., followed by cooling to set the shape of thestent, an expanded wall thickness is approximately 1 mm, an outerdiameter is approximately 20 french, and an expanded length isapproximately 20 mm.

Other Embodiments

In some of the embodiments of any of the above stents, only a portion orportions of the stent (e.g., the portion(s) having an expanded position)may be composed of the polymer. The remainder of the stent may be, forexample, composed of a non-polymeric material (e.g., a metal or metalalloy, e.g., Ni/Ti alloy). Moreover, the stent may be composed ofmultiple layers of materials, for example, by co-extruding the layerswhen making an elongated element. The stent may be a multiple segmentstent.

The polymer in any of the above stents may be a blend of polymers, forexample, miscible blends of a semicrystalline polymers with an amorphouspolymer. For those blends that are miscible at the molecular level, asingle glass transition results, without broadening. Additionally, insuch miscible blends the equilibrium crystallinity (which controls theplateau modulus between T_(g) and T_(m) where shape fixing is performed)also changes dramatically and systematically with blend composition;i.e., relative levels of each component.

Polymers blends with a relatively high modulus in the fixed state atroom temperature, having a tunable and sharp transition, the permanentshape of which can be remolded repeatedly above certain meltingtemperatures are prepared by the blending of crystalline polymers (C′)with amorphous polymers (A′), such that they are a single miscible phasein the molten state (allowing processing to stress-free native states)but crystalline to a limited and tailored extent and which furthervitrify on cooling to room temperature. The recovery of the polymerblend may be fast, for example, within seconds. Examples for (C′)include poly(vinylidene fluoride) (PVDF) (T_(g)=−35° C., T_(m)=175° C.),polylactide (PLA) (T_(g)=56° C., T_(m)=165° C.), poly(hydroxy butyrate),poly(ethylene glycol) (PEG), polyethylene, polyethylene-co-vinylacetate, poly(vinyl chloride) (PVC), and poly(vinylidene chloride)(PVDC) and copolymers of poly vinylidene chloride (PVDC)/poly vinylchloride (PVC). Examples for (A′) include poly(vinyl acetate) (PVAc)(T_(g)=35° C.), poly methyl acrylate (PMA), poly ethyl acrylate (PEA),atactic poly methyl methacrylate (aPMMA), isotactic poly methylmethacrylate (iPMMA), syndiotactic poly methyl methacrylate (sPMMA), andother poly alkyl methacrylates.

In some preferred embodiments formed from two miscible polymer blends,the blend is prepared by mixing amorphous poly(vinyl acetate) (PVAc)(T_(g)=35° C.) with semicrystalline polylactide (PLA) (T_(g)=56° C.,T_(m)=165° C.) or poly(vinylidene fluoride) (PVDF). The polymers showcomplete miscibility at all blending ratios with a single glasstransition temperature, while crystallization (exclusive of PVAc) ispartially maintained. The T_(g)'s of the blends are employed as thecritical temperature for triggering the shape recovery while thecrystalline phases serve as physical crosslinking sites for elasticdeformation above T_(g), but below T_(m).

The preferred blends are formed from poly vinyl acetate(PVAC) andpoly(lactic acid) (PLA) or poly(vinylidene fluoride) (PVDF). However,examples of other suitable blends include the pair PVDF/PMMA and ternaryblends of PVDF/PMMA/PVAc. The PMMA and the combination of PMMA/PVAcserve the same role as PVAc in the blends as have been previouslydescribed. An advantage of adding PMMA is that the critical temperaturecan be increased arbitrarily to about 80° C. and the room temperaturemodulus can also be increased. The PVDF may be substituted bypoly(vinylidene chloride) (PVDC), by copolymers of poly(vinylidenechloride/ply(vinyl chloride), or by any “C” polymer vide supra.

It has further been found that blending poly(vinyl chloride) withpoly(butyl acrylate) or poly(butyl methacrylate) (PVC/PBA) has certainadvantages. In the PVDF/PVAc case, PVAc simultaneously lowers thecrystallinity of PVDF while increasing T_(g). PVC may serve the samerole as PVDF, but it already has a low degree of crystallinity, but arelatively high T_(g) (˜80° C.). Thus in this embodiment, the secondcomponent (PBA) serves only the role of decreasing T_(g). This can alsobe achieved with small molecule plasticizers, most notablydioctylphthalate (DOP), but is preferred to use a biocompatiblepolymeric plasticizer for intended implantable applications. The rangeof PBA compositions is 10-40%, with 20% being the most advantageous,yielding a T_(g)˜40° C.

Melt blending of PLA/PVAc and PVDF/PVAc of varying blend ratios wasperformed in a 30 ml Brabender mixer. The mixer was equilibrated atT=180° C. for 5 minutes after which the mixer blade rotation wasadjusted to 25 RPM and the premixed polymers pellets added to thechamber over the course of 1 minute. The polymers were mixed for 10minute to ensure good dispersion. Nitrogen was purged through thechamber to mitigate potential oxidative degradation during mixing. Aftermixing, the blend was taken out of the chamber, cooled to roomtemperature, and then pressed between heated platens of a Carver pressat 180° C. for 5 minutes under a load of 8 metric tons. A spacer wasused to control the thickness of the film and rapid cooling to roomtemperature was carried out. The films thus formed were used for thesubsequent thermal and mechanical characterization.

The TGA results demonstrated that both PLA and PVAc are stable forT<300° C. Above this temperature PLA degrades completely (no charyield), while the PVAc degrades to yield an intermediate char yield of25 wt % for 375<T<425° C. but complete degradation above 450° C. Blendprocessing and thermal and dynamic mechanical analyses (DSC and DMA)were performed below 250° C., to completely avoid degradation.

The crystallization behavior of semicrystalline PLA was investigated viaDSC. The PLA samples were first heat pressed at 180° C. for 10 minutesand then quenched to room temperature with water cooling. One sample wasdirectly analyzed by DSC, while another was first annealed at 110° C.(=½(T_(g)+T_(m))) for 1 hour to reach an equilibrium level ofcrystallinity. FIG. 36 shows a comparison of thermal behavior for thesetwo samples. It was observed that quenching the PLA melt results in alow degree of crystallinity and virtually no recrystallization onheating, both indicating slow crystallization Annealing at 110° C. for 1hour results in significant crystallization evidenced by a large meltingendotherm at T=155° C. The melting temperature did not shiftdramatically due to annealing, but the endotherm shape did change.Complementary WAXD experiments yielded the same conclusions.

The crystallization behavior selected of polymer blends was alsoanalyzed. All of the samples were heat pressed at 180° C. for 10 minutesand then annealed at 110° C. for 1 hour before thermal analysis,providing a standard condition for extensive crystallization. FIG. 37summarizes the first DSC heating trace of the samples measured afterannealing. The results indicate that PVAc itself is amorphous (thoughwith large physical aging content) but that incorporation of PLA leadsto crystallization in proportion to the PLA wt-%. Also, the peakendotherm positions (melting transitions) shift slightly to highertemperatures with increasing PLA content. Quenching these samples toT=20° C. and reheating to 200° C. again showed clearly that singleT_(g)'s are observed and that crystallization can be largely suppressed.Importantly for shape memory, the single glass transition events werenot broadened in the blends relative to the pure components, suggestingthat the amorphous phase was quite homogeneous for all of the blends.The observed T_(g) values are plotted in FIG. 38 along with the best fitwith the Fox equation, showing slight positive deviation. This leads toa conclusion that strong interactions between the two polymers thatreduces free volume of the polymer blends and hence, increased glasstransition temperature relative to the Fox equation prediction hasoccurred.

In order to elucidate the effect of PVAc on the degree of crystallinityand the crystal structures, the crystalline diffraction patterns wereobserved via wide-angle x-ray diffraction. The results indicate that thePVAc phase has only an amorphous halo, thus being totally amorphous,while the PLA exhibits three very strong diffraction peaks at 2θ=22.3°,25.0° and 28.6°, corresponding to d-spacings of 5.92, 5.29, and 4.64 A°,respectively. Upon addition of PVAc, all of the peak intensities weredepressed, but the peak positions remained essentially unchanged.Consistent with the DSC results, the degree of crystallinity increasesin proportion to PLA addition. From the peak width at half height, itwas found that the crystalline lamellae size did not decrease, althoughthe degree of crystallinity decreased, with increasing PVAc content.This means that the decrease in crystallinity and depression of themelting transitions are not due to a change of crystal size, but rathermay be due to a thinning of the lamellae thickness or to a decrease ofthe crystal concentration.

The storage modulus of the polymer blends was also measured using DMTA,first investigating the effects of annealing on the storage modulus.Below their glass transition temperatures, T_(g), both samples exhibitsimilar high storage moduli (3 GPa), as well as similar softeningpoints. When heated above T_(g), the storage modulus of thermallyquenched samples decreases sharply to about 2 MPa; however, furtherincreasing the temperature induces a modulus increase attributed torecrystallization of the samples at higher temperatures. This alsoproved that the sample is not in an equilibrium state and that itsmechanical properties in the rubbery region depend on thermal history.To reach equilibrium, the sample was annealed at 110° C. for 1 hour aspreviously described for DSC analyses. The storage modulus above T_(g)shifts to about 200 MPa until melting, the increase being due to anincrease of the degree of crystallinity on annealing to tune the rubberymodulus at equilibrium state. PLA was blended in different proportionsto PVAc and annealed as above. Storage moduli for such blends weremeasured and the results are plotted in FIG. 39. It can be seen that,below T_(g), all of the samples show similar large moduli while aboveT_(g) the moduli decrease to a plateau whose magnitude depends oncrystallinity and thus PLA content. This trend is in accordance withthat of DSC and XRD, and can be explained by the fact that the increaseof storage moduli came from the physical crosslinking formed by crystalsand the filler effect of the high modulus crystalline phase.

Stress-free shape memory tests were carried out in hot water at 65° C.,with an annealed sample composed of 30% PLA. The results show that thesample features quick and complete shape memory behavior: the samplerecovers to the original shape (straight bar) within 10 seconds, withmost of the recovery being accomplished within the first severalseconds.

The same characterizations were carried out on the blends of PVDF andPVAc as above disclosed. The TGA and DSC results show that PVDF is alsothermally stable up to 300° C., and the mixtures form only one glasstransition, the values fall between the Tgs of the two homopolymers andchanges with changing composition. At the same time, the melting pointsand the degrees of crystallinity were depressed with the incorporationof amorphous PVAc.

The storage moduli of the blends, which give the rigidity of thematerials, were also measured. The results are similar to those of thePLA/PVAc blends, the PVDF/PVAc blends being very rigid below thecritical temperatures (T_(g)), and featuring a sharp modulus changes atthe Tg to a plateau modulus ranging from several MPa to tens of MPa,depending on the degree of crystallinity of the blends. These plateaumoduli can be tuned by adjusting the degree of crystallinity of theblend, that is, adjust the blend composition.

The polymer in any of the above stents may be bioabsorbable ornon-bioabsorbable. Bioabsorbable polymers include, for example,polyurethanes and polyurethane copolymers such as those described abovewith the general formula (directly below), where X/Y is, for example, 1to 20, n is, for example, 2 to 1000, and the total degree ofpolymerization m is, for example, 2 to 100

The bioabsorbability of the polymers is enhanced by copolymerization ofpolyurethane and POSS with suitable monomers. Examples of suitablemonomers include caprolactone, ethyleneglycol, ethylene oxide, lacticacid, and glycolic acid. The copolymers from these monomers canhydrolyze and cleave the polymer linkage.

Other embodiments of stents can also be formed to include materialsdescribed above. In some embodiments, an implantable medical stent maybe delivered into the body using a catheter. The stent can be deliveredin a small diameter form and then expanded at a treatment site bytriggering a shape change (for example, by heat application) caused bythe shape memory properties of the polymer. The stent can also beexpanded by a mechanical expander such as an inflatable balloon of thetype used on an angioplasty catheter.

In some embodiments, the stent is sized (e.g., an expanded innerdiameter of about 2 mm to about 20 mm) and configured for use in thevascular system, particularly the coronary arteries, and implanted afteror simultaneously with an angioplasty procedure to maintain an openlumen and reduce restenosis. Vascular stents are described in U.S.Provisional Application No. 60/418,023, which is hereby incorporated infull by reference. For example, a stent for coronary use can have aninitial diameter of about 2 mm, an expanded diameter of about 4 mm, anda wall thickness of about 0.005 mm to 0.1 mm. Other exemplaryapplications include neuro, carotid, peripheral, and vasculature lumens.The vascular stent can be bioabsorbable or non-bioabsorbable.

In other embodiments, a stent, e.g., a bioabsorbable or anon-bioabsorbable stent, is constructed for use in nonvascular lumens,such as the esophagus, ureteral, biliary, or prostate.

In other embodiments, the stent is conductive to allow an electricalcurrent to pass through the stent, for example, to deliver electricityto an area of the body or to trigger, for example, a physical change inthe stent, for example, a change in the diameter of the stent.

In still other embodiments, the stent, for example, of FIGS. 4, 23 and26, is made porous by, for example, adding a chemical foaming agent tothe polymer from which the stent is made during the production theplastic strand. In some implementations, the stent is porous andincludes a medicament. The initial porosity of the stent can be reduced,for example, by the application of heat and pressure before deploymentin the body. Upon deployment of the stent in the body, the porosity isincreased by a triggering event, for example, the application of heat tothe stent at the desired site of treatment.

What is claimed is:
 1. A product comprising: an endoprosthesis having abody formed of a non-bioabsorbable polymer comprising a cross-linkedpolycyclooctene, the polycyclooctene having been cross-linked while theendoprosthesis is in an expanded position to provide shape memory to theendoprosthesis, the endoprosthesis being designed for delivery into abody lumen and being in a collapsed state so that the endoprosthesis canbe delivered into the body lumen, and wherein the endoprosthesis iscapable of reverting from the collapsed state to the expanded state whenwarmed after delivery to a body lumen.
 2. The product of claim 1,wherein the body has a wall having a wall thickness and wherein the wallthickness is maintained if the endoprosthesis reverts from the collapsedstate to the expanded state.
 3. The product of claim 2, wherein thechange in the wall thickness is less than about 1%.
 4. The product ofclaim 2, wherein the wall thickness is from about 0.005 mm to about 5mm.
 5. The product of claim 1, wherein the endoprosthesis is a stent. 6.The product of claim 1, wherein the endoprosthesis is capable ofreverting from the collapsed state to the expanded state at atemperature below 60° C.
 7. The product of claim 1, wherein theendoprosthesis is capable of reverting from the collapsed state to theexpanded state at a temperature above 37° C.
 8. The product of claim 1,wherein only a portion of the endoprosthesis is in a collapsed statethat is capable of reverting to an expanded state when warmed.
 9. Theproduct of claim 1, wherein the polycyclooctene has been chemicallycross-linked.
 10. The product of claim 1, wherein the endoprosthesis hasa tubular shape.
 11. The product of claim 1, wherein the polycyclooctenehas been radiation cross-linked.
 12. The product of claim 1, wherein thepolycyclooctene exhibits a plateau in its plot of storage modulus as afunction of temperature.
 13. The product of claim 1, wherein theendoprosthesis further includes radiopaque agent.
 14. The product ofclaim 1, wherein the endoprosthesis further includes a drug.
 15. Theproduct of claim 1, wherein the body of the endoprosthesis ismetal-free.
 16. The product of claim 1, wherein the body of theendoprosthesis consists of the cross-linked polycyclooctene.
 17. Aproduct comprising: a stent having a metal-free body formed of anon-bioabsorbable polymer comprising a cross-linked polycyclooctene, thepolycyclooctene having been cross-linked while the stent is in anexpanded position to provide shape memory to the stent, the stent beingdesigned for delivery into a body lumen and being in a collapsed stateso that the stent can be delivered into the body lumen, and wherein thestent is capable of reverting from the collapsed state to the expandedstate when warmed after delivery to a body lumen.
 18. The product ofclaim 17, wherein the wall thickness is from about 0.005 mm to about 5mm.
 19. A product comprising: a stent having a body consisting ofnon-bioabsorbable cross-linked polycyclooctene, the polycyclooctenehaving been cross-linked while the stent is in an expanded position toprovide shape memory to the stent, the stent being designed for deliveryinto a body lumen and being in a collapsed state so that the stent canbe delivered into the body lumen, and wherein the stent is capable ofreverting from the collapsed state to the expanded state when warmedafter delivery to a body lumen.
 20. The product of claim 19, wherein thewall thickness is from about 0.005 mm to about 5 mm.